This invention relates to an improved self-shielded gradient coil assembly for a conventional magnetic resonant (MR) system wherein inner and outer gradient coil windings are wound on inner and outer concentric fiber-reinforced plastic (FRP) cylinders with an annular space therebetween filled with a filler material and, more particularly, to a concrete filler material comprising cement and a selected aggregate material.
It is well known in MR imaging to employ a self-shielded gradient coil assembly (also known as a self-shielded gradient coil set) to generate the required gradient magnetic fields, i.e., the X-, Y-, and Z-gradient fields, where X, Y and Z represent the respective axes of a three-dimensional Cartesian coordinate system. Generally, a self-shielded gradient coil assembly has inner and outer gradient coil windings wrapped around respective, inner and outer cylindrical coil forms disposed in spaced-apart coaxial relationship within the bore of an associated MR main magnet. Each of the coil windings is actuated or energized by a corresponding current to produce a magnetic field, the inner and outer coil-windings being designed in relation to each other so that their fields combine within the bore of the magnet to produce a resultant magnetic field comprising respective ones of the X-, Y- and Z-gradient fields. The fields of the two coils substantially cancel each other outside of the outer coil form, so as not to modify or alter the main magnetic field. A gradient coil set, or assembly, is described, for example, in commonly assigned U.S. Pat. No. 4,737,716, issued Apr. 12, 1988 to Roemer et. al., and for which re-examination certificate B1, U.S. Pat. No. 4,737,716 was issued.
It is currently common practice in MR imaging to place the inner coils for X-, Y-and Z-gradient coil sets on a single inner coil form, and to place the corresponding outer X-, Y- and Z-coils on a single outer coil form which is in coaxial relationship with the inner coil form. Thus, the inner and outer coils, or coil windings, of each coil set, or assembly, are in radially spaced apart, coaxial relationship with each other. In a typical such arrangement, each coil of the Z-gradient coil set comprises a wire helically wound around one of the coil forms. Each coil of the X-gradient coil set comprises a xe2x80x9cfingerprint coilxe2x80x9d or the like, which is etched or otherwise formed on a sheet or board of copper, each such sheet being wrapped around a coil form over the Z-gradient windings. The Y-gradient coils likewise comprise fingerprint coils formed on copper sheets, one such sheet being wrapped around each coil form over the sheet containing the X-gradient windings, but in orthogonal relationship therewith. See, for example, U.S. Pat. No. 4,646,024xe2x80x94Schenck et al., issued Feb. 27, 1987 and assigned to the common assignee herewith.
When current pulses are applied to the gradient coil windings contained within the static field of the MR main magnet, mechanical forces are created that tend to displace the coils relative to their respective coil forms. Accurate positioning of the coils relative to their forms is critical for proper generation of gradient fields; therefore, such mechanical forces must be opposed to prevent displacement of the coils relative to each other and to the respective forms thereof and, moreover, to maintain the integrity of the structural relationship thereby to suppress mechanical vibrations which could damage the coils and also generate a much higher level of noise in the surrounding environment.
The high current levels employed in conventional gradient coils produce significant heat proximate to the coils. The heat must be conducted away from the coils and the magnet bore region to prevent damage to the coils and related structure, to avoid unwanted changes in the main magnetic field due to heating of magnet components, and to prevent unacceptable heating of a patient or other subject in the bore.
Commonly assigned U.S. Pat. No. 5,570,021 issued Oct. 29, 1996 to Dachniwskyj et al. discloses a gradient coil support assembly for an MR imaging system which addresses the above problems resulting from energization of the gradient coil windings. The assembly includes a first cylindrical coil form disposed to support a first gradient coil of a gradient coil set, and a second cylindrical coil form disposed to support a second gradient coil of the set, the second coil form being positioned in coaxial spaced-apart relationship with the first coil form and forming an annular, cylindrical space therebetween. A stiffening cylinder is positioned in the space between the first and second coil forms to divide the annular space between the first and second coil forms into a first volume located between the stiffening cylinder and the first coil form, and a second volume located between the stiffening cylinder and the second coil form. Adhesive material, such as an epoxy, is introduced into the first and second volumes to bond both the first and second coil forms to the stiffening cylinder and thereby hold the first and second gradient coils in rigid, fixed relationship with respect to each other. With the second coil form having a larger diameter than the first coil form, a first flexible cooling tube is helically positioned around the first coil form and located in the first volume, and a second flexible cooling tube is helically positioned around the stiffening cylinder and located in the second volume. A circulating unit directs coolant through the cooling tubes to remove heat from regions proximate to the gradient coils. A layer of filament, such as a fiberglass element, may be wrapped around each coil form and the gradient coil winding thereon to provide additional support for tightly holding each coil winding on its respective coil form. The stiffening cylinder thickness may be selected in relation to the spacing between the first and second coil forms to provide respective dimensions for the first and second volumes selected to optimize curing of the epoxy therein. The dimensions are intended to provide a specified minimum curing time. Also, the epoxy in each volume is caused to cure or harden uniformly, i.e., the epoxy at each point in the volume hardens at very nearly the same time.
While reasonably effective in providing the bonding functions, epoxy does not afford adequate suppression of vibration and noise. Decreasing vibrations is important, particularly as current levels in the gradient coils increase, to afford a reduction in the accompanying acoustic noise during operation of the MR systems.
An improved filler material for a self-shielded gradient coil assembly of an MR system makes the assembly stiffer and/or more highly damped, thereby more effectively suppressing current-pulse-generated vibrations and consequent acoustic noise, while an attaching and supporting structure for such gradient coil assembly is capable of suppressing vibrations and opposing coil displacement resulting from strong mechanical forces applied to the coils as a result of high currents in the coils while the gradient coil assembly is situated in a static magnetic field.
The attaching and supporting structure of the invention reduces the sizes of the respective annular volumes, or spaces, into which epoxy is introduced, enhancing curing uniformity and significantly reducing curing time for the epoxy. The attaching and supporting structure also allows for the assemblage therein of water cooling tubes which greatly reduce undesirable heating resulting from operation of the coils at high current levels and also significantly reduce noise produced in the gradient coil assembly when operated at high current levels.
More particularly, a concrete material, preferably a conglomerate of Portland cement and one or more selected aggregates, affords more effective suppression of vibration and noise, relative to the suppression thereof afforded by a conventional epoxy filler, and at low cost. The concrete may be introduced directly into the annular cylindrical space between inner and outer cylindrical, concentric FRP cylinders having respective helices of cooling tubes affixed on their respectively opposing, outer and inner surfaces. Alternatively, a concrete cylinder with a hollow annulus, i.e., a cylindrical concrete sleeve, of appropriate inner and outer diameters and with radial thickness less than that of the annulus formed between the concentrically-positioned inner and outer FRP cylinders may be inserted coaxially into the annulus, thereby defining inner and outer annular volumes, or spaces, in which respective water cooling tubes are supported. These annular volumes are then filled with a conventional thermal epoxy. Testing has shown that a significant reduction in noise levels is achieved through use of a concrete filler material relative to that afforded by conventional epoxy filler materials.